## Forces in Action

**Force**

**- interaction between two objects, fields or charges measured in Newtons**

**Newton - force required to accelerate a mass of 1 kg at a rate of 1 m/s^2**

## Resolving Forces

As Galileo proposed,

Force is the product of these two. As mass is a scalar and acceleration a vector they give a vector.

Giving:

Force (N)

**mass and acceleration are two separate entities**Force is the product of these two. As mass is a scalar and acceleration a vector they give a vector.

Giving:

Force (N)

**F = ma**mass (kg) x acceleration (m/s^2)In objects acceleration is generally the changing value as

**mass remains constant**. This however doesn't happen in:- Rockets - because the fuel is reducing and so the mass reduces
- Approaching the speed of life - The mass of an object
**increases**

Usng the Force equation weight can be calculated by:

Weight (N)

This part explains why a hammer falls faster than a feather

Weight (N)

**W = mg**mass (kg) x gravity (m/s^2)This part explains why a hammer falls faster than a feather

## Non-Constant acceleration

Non-constant acceleration is the reality of the world because objects are subject to drag

Drag depends on:

They will generally ask what happens when someone goes skydiving - the forces in affect are shown below

Drag depends on:

- Velocity
- Cross-sectional area
- Roughness of shape
- Shape

They will generally ask what happens when someone goes skydiving - the forces in affect are shown below

**Speed time graph for non-constant acceleration**

## Drag Force Equation

Fd = Drag Force

p = density of air

Cd = Drag coefficient of object

A = Frontal area

v = velocity

This explains why the acceleration gets harder as terminal velocity is approached.

For when the

This also explains why when the parachute on a skydiver is opened the velocity decreases as the drag force increases dramatically

p = density of air

Cd = Drag coefficient of object

A = Frontal area

v = velocity

This explains why the acceleration gets harder as terminal velocity is approached.

For when the

**velocity doubles**the**drag force quadruples**This also explains why when the parachute on a skydiver is opened the velocity decreases as the drag force increases dramatically

## Equilibrium

Equilibrium is when an object's

**is zero (like at terminal velocity)**__net force__**Use vector triangles as shown in**

__Motion__to work out forces in equilbrium## Centre of Gravity

**The point where all weight is said to act (where all the moments are said to act)**

To find the centre of gravity of a uniform shape is simple as it's where all the diagonals cross.

To find the centre of gravity of a non-uniform shape is slightly different. You do it by hanging a

To find the centre of gravity of a non-uniform shape is slightly different. You do it by hanging a

**plumline**from different datem points drawing where it falls and finding where they cross.## Moments

**A moment is the turning effect of a single force acting perpendicular to a beam**

**The**

__principle of moments__states that in a__uniform beam__in equilibrium balanced on a pivot; the

**sum of the anti-clockwise****forces acting**

__perpendicular__**to the beam**

__equal__the__sum of clockwise__forces__perpendicular__to the beamA moment is measured in

This gives the equation where x is the distance:

**Newton metres (**as it is a measure of a force acting at a perpendicular distance from a stated point.__Nm__)This gives the equation where x is the distance:

**sum of clockwise moments Cx = Ax sum of anti-clockwise moments**In the above diagram

**(F1 x d1) = (F2 x d2) + (F3 x d3)**## Torque

Torque occurs when there is a

**couple**on a beam. This is when two moments work in the same rotational direction at the same perpendicular distance from a point.**Couple = Fs**

## Density

**(density kg/m^3)**

**ρ =**

__m__(mass kg)**v (volume m^3)**

## Pressure

**(pressure Pa or N/m^2) p =**

__F__(Force N)**A (Area m^2)**

**Pressure in a liquid**

**(pressure Pa) p = h**

**ρg (height submerged m x density kg/m^3 x gravity m/s^2)**

## Car Stopping Distances

Has two parts -

**thinking distance**and**braking distance****s = ut +**__-u^2__**2a**Many different things affect the stopping distance like road condition, concentration of driver etc.

## Car Safety

Force felt during a collision is shown in this equation (

**this isn't given in the exam**)**F =**__m(v-u)__**t**## Crumple Zones

By

**increasing the time of the impact**this**decreases the force**so crumple zones are designed to increase the time of the impact## Seat Belts

Seat belts prevent you from

**hitting hard object****s**in the car by keeping secure in one position## Airbags

Airbags are also designed to increase the time of the impact as they support body parts not protected by the seat belt. Without a seat belt they are dangerous as whilst the body goes forward it hits the airbag traveling towards it, which decreases the time of impact.

The system for an airbag is shown below:

The system for an airbag is shown below:

When the mass experiences

**10g**(this is only experienced during an accident) it gains enough energy to**complete the circuit**by touching the connector. This creates the**spark**needed in the chemical mixture to set off a violent reaction which releases**Nitrogen gas**which is what expands the airbag.## GPS

Global Positioning Systems work via the principle of

**Trilateration****The GPS in a car can work out where it is by picking up**

**three satellites signals**. Each satellite is in a fixed position above the Earth (in**geostationary orbit**) and this means that the car can work out how far it from each satellite by picking up the specific satellite signal and working out how far it has travelled. It then plots where it is on a map.